Weather & climate change: how to interpret our past in order to prepare for our future

Summary: Nate Silver’s new 538 project has started with a bang by featuring Roger Pielke Jr telling us about the research showing the trends in damage from natural disasters. It’s an important issue, reminding us of our infrastructure’s vulnerability to weather — weather of the kind we’ve seen in the past, and the probably worse we’ll see in the future. Previous posts discussed the hostile response by activists, illustrating how the debate about the public policy response to climate change has collapsed into a cacophony. Here’s an attempt to understand the issues Pielke has raised.

Meanwhile, climate science moves on. We will get definitive answers eventually, but perhaps to late for effective policy action. Hence the debate.

Lots of chaff has been tossed into the air to hide the peer-reviewed literature on this subject, which largely confirms Pielke’s analysis. Also seldom mentioned by Pielke’s critics is what the IPCC has to say on the subject.

Here is the #1 flawed reasoning you will have seen about this question: it is the classic confusion between absence of evidence and evidence for absence of an effect of global warming on extreme weather events. Sounds complicated? It isn’t.

The two most fundamental properties of extreme events are that they are rare (by definition) and highly random. These two aspects (together with limitations in the data we have) make it very hard to demonstrate any significant changes. And they make it very easy to find all sorts of statistics that do not show an effect of global warming – even if it exists and is quite large.

The fundamental issue here is not even one of attribution -– rather it is detecting a meaningful change. For record high temperatures, it is fairly straightforward to expect more records as average temperatures increase. But this is much more difficult for drought, hurricanes and other extreme weather events. The RC post argues that physical reasoning is sufficient, e.g. warmer sea surface temperatures drive more intense hurricanes. The RC post concludes:

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… While statistical studies on extremes are plagued by signal-to-noise issues and only give unequivocal results in a few cases with good data (like for temperature extremes), we have another, more useful source of information: physics. For example, basic physics means that rising temperatures will drive sea levels up, as is in fact observed. Higher sea level to start from will clearly make a storm surge (like that of the storms Sandy and Haiyan) run up higher. By adding 1+1 we therefore know that sea-level rise is increasing the damage from storm surges – probably decades before this can be statistically proven with observational data.

… With good physical reasons to expect the dice are loaded, we should not fool ourselves with reassuring-looking but uninformative statistics. Some statistics show significant changes – but many are simply too noisy to show anything. It would be foolish to just play on until the loading of the dice finally becomes evident even in highly noisy statistics. By then we will have paid a high price for our complacency.

The RC post seems in part motivated by Roger Pielke Jr’s 538 piece:

… If an increase in extreme weather events due to global warming is hard to prove by statistics amongst all the noise, how much harder is it to demonstrate an increase in damage cost due to global warming? Very much harder! A number of confounding socio-economic factors clouds this issue which are very hard to quantify and disentangle.

Some factors act to increase the damage, like larger property values in harm’s way. Some act to decrease it, like more solid buildings (whether from better building codes or simply as a result of increased wealth) and better early warnings. Thus it is not surprising that the literature on this subject overall gives inconclusive results. Some studies find significant damage trends after adjusting for GDP, some don’t, tempting some pundits to play cite-what-I-like.

The fact that the increase in damage cost is about as large as the increase in GDP (as recently argued at FiveThirtyEight) is certainly no strong evidence against an effect of global warming on damage cost. Like the stranger’s dozen rolls of dice in the pub, one simply cannot tell from these data.

I’m not comfortable with Pielke’s assertion that climate change has played no role in the observed increase in damages from natural hazards; I don’t see how the data he cites support such a confident assertion.

There is an even more significant problem with Pielke’s analysis. In a nutshell, he addresses trend detection when what we need is event risk assessment. The two would be equivalent if the actuarial data was the only data available pertaining to event risk. But that is far from the case; we often have much more information about risk.

This does not mean that there is no underlying change in the risk, and the priors we have in this case point to a significant increase in such risk. One would be foolish to make plans that have to deal with U.S. hurricane risk without accounting for the evidence that the underlying risk is increasing, whether or not actuarial trends have yet emerged at the 95 percent confidence level.

This is particularly so when one accounts for another form of prior information: theory and models. While some disagreement remains about projections of the weakest storms, which seldom do much damage, both theory and models are now in good agreement that the frequency of high category hurricanes should increase, as should hurricane rainfall and the flooding it produces.

Or is he? What this analysis is illustrating, I think, is the time at which virtually all models show an increased trend. So, yes, this is the time at which we would almost certainly see an increased trend (assuming the assumptions are appropriate) but it doesn’t tell you how likely it is to see an increased trend at an earlier time.

… It may well be more than 200 years before we’re virtually certain to be able to detect an increased trend in normalised losses, but there’s a 50% chance that it will occur before 2100 and about a 15% chance that it will occur before 2045. I would certainly argue (assuming I haven’t made some silly mistake) that this is relevant. Surely it’s not simply when we’re virtually certain to detect an increased trend, but also how likely such an increased trend is in the coming decades.

Existence of potential is not evidence of existence of outcome. For examples:

The potential for increased atmospheric temperature to retain increased water is not evidence that the atmosphere contains increased water.

The potential for increased atmospheric water content is not evidence that extreme rainfall events will occur.

The potential for increased atmospheric temperature to lead to increased strength and frequency of topical storms is not evidence that topical storms have increased in frequency and strength.

Radiative energy exchange balance at the top of the atmosphere implies energy-exchange balance ( not limited to radiative exchange ) at all the interfaces between the sub-systems within the Earth’s climate systems. The potential for radiative energy exchange balance at the top of the atmosphere is not evidence of energy exchange balance at sub-systems interfaces.

(3) Judith Curry’s reflections on this debate

There are two fundamental issues here regarding reasoning about the impact of AGW on extreme weather events:

the utility of physical reasoning and models in assessing this impact

the choice of a null hypothesis

When it comes to extreme weather events, climate models are inadequate at simulating them; exercises in attribution that turn the anthropogenic impact on and off are of little use if the models have inherent problems in simulating the extreme events of interest.

With regards to the physical reasoning, as an example I’ll tackle the canard whereby global warming increases hurricane intensity (for a review of this topic, see this previous post). Simple physical reasoning suggests that increased sea surface temperatures will increase hurricane intensity.

But what about circulation changes associated with global warming, that might provide more or fewer El Nino events? That might change wind shear? That might change subtropical dust emissions? Etc. There is no way to reason through the complex changes that might occur to complex atmosphere and ocean circulation systems and their impact on extreme events. There is observational increase of hurricane intensity in the North Atlantic and the North Indian Ocean since 1980. In the Atlantic, with such a short data set, there is no way to sort out whether this increase is associated with the transition to warm phase of the AMO, or to AGW.

For #1, the null would be rejected if you find evidence of a human influence. In the absence of such evidence, #1 is not rejected. This is what RP Jr argued. For #2, the null would be rejected if there is evidence of no influence. RC and Emanuel essentially that the data is insufficient, so they argue from ‘physics’ and state that there is no evidence of absence.

To me, the ‘no evidence of absence’ argument is rather fatuous given that simple thermodynamical reasoning is not really useful in elucidating the impacts of AGW on extreme weather events.

So, how to answer the question Does global warming make extreme weather events worse? Just say we don’t know. There is no evidence even of a trend in most extreme events; attributing any trend to AGW is extremely difficult. We can say that it is possible that global warming will make extreme weather events worse, but the evidence is theoretical.

Bottom line is that RP Jr took a very defensible position in his essay. Kerry Emanuel’s response was interesting, but he puts forward a theoretical scenario of worsening extreme events with global warming. The theoretical scenario is convincing to people who adopt as a null hypothesis and starting point for their reasoning that AGW is worsening extreme weather events. It is not convincing to people looking for empirical evidence.

—————————– End of Professor Curry’s article —————————–

(3) About Judith Curry

Judith Curry is Professor and Chair of the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology. She is also President and co-owner of Climate Forecast Applications Network (CFAN). Prior to joining the faculty at Georgia Tech, she served on the faculty of the University of Colorado, Penn State University and Purdue University.

She serves on the NASA Advisory Council Earth Science Subcommittee and the DOE Biological and Environmental Science Advisory Committee. She recently served on the National Academies Climate Research Committee and the Space Studies Board, and the NOAA Climate Working Group.

She is a Fellow of the American Meteorological Society, the American Association for the Advancement of Science, and the American Geophysical Union.

Her views on climate change are best summarized by her recent Congressional Testimony:

The increasing impact of natural disasters over recent decades has been well documented, especially the direct economic losses and losses that were insured. Claims are made by some that climate change has caused more losses, but others assert that increasing exposure due to population and economic growth has been a much more important driver. Ambiguity exists today, as the causal link between climate change and disaster losses has not been addressed in a systematic manner by major scientific assessments.

Here I present a review and analysis of recent quantitative studies on past increases in weather disaster losses and the role of anthropogenic climate change. Analyses show that although economic losses from weather related hazards have increased, anthropogenic climate change so far did not have a significant impact on losses from natural disasters. The observed loss increase is caused primarily by increasing exposure and value of capital at risk.

This finding is of direct importance for studies on impacts from extreme weather and for disaster policy. Studies that project future losses may give a better indication of the potential impact of climate change on disaster losses and needs for adaptation, than the analysis of historical losses.

Climate change is likely to lead to an increase in the frequency and/or intensity of certain types of natural hazards, if not globally, then at least in certain regions. All other things equal, this should lead to an increase in the economic toll from natural disasters over time. Yet, all other things are not equal since affected areas become wealthier over time and rational individuals and governments undertake defensive mitigation measures, which requires normalizing economic losses if one wishes to analyze trends in economic loss from natural disasters for detecting a potential climate change signal.

In this article, we argue that the conventional methodology for normalizing economic loss is problematic since it normalizes for changes in wealth over time, but fails to normalize for differences in wealth across space at any given point of time. We introduce an alternative methodology that overcomes this problem in theory, but faces many more problems in its empirical application. Applying, therefore, both methods to the most comprehensive existing global dataset of natural disaster loss, in general we find no significant upward trends in normalized disaster damage over the period 1980–2009 globally, regionally, for specific disasters or for specific disasters in specific regions.

Due to our inability to control for defensive mitigation measures, one cannot infer from our analysis that there have definitely not been more frequent and/or more intensive weather-related natural hazards over the study period already. Moreover, it may still be far too early to detect a trend if human-induced climate change has only just started and will gain momentum over time.

It is a widely held view that climate change arising from human activity is increasing the cost of natural disasters. This perception is false. While it is undeniable that the economic cost of natural disasters is rising rapidly, it is doing so because of growing concentrations of population and wealth in disaster-prone regions. So far studies of long-term insurance or economic disaster loss histories caused by extreme weather — tropical cyclones, floods, bushfires (wildfires) and storms — have been unable to identify a contribution from human-induced climate change. This is true for many different natural perils and across jurisdictions.

A holistic perspective on changing rainfall-driven flood risk is provided for the late 20th and early 21st centuries. Economic losses from floods have greatly increased, principally driven by the expanding exposure of assets at risk. It has not been possible to attribute rain-generated peak streamflow trends to anthropogenic climate change over the past several decades.

Projected increases in the frequency and intensity of heavy rainfall, based on climate models, should contribute to increases in precipitation-generated local flooding (e.g. flash flooding and urban flooding). This article assesses the literature included in the IPCC SREX report and new literature published since, and includes an assessment of changes in flood risk in seven of the regions considered in the recent IPCC SREX report — Africa, Asia, Central and South America, Europe, North America, Oceania and Polar regions.

Also considering newer publications, this article is consistent with the recent IPCC SREX assessment finding that the impacts of climate change on flood characteristics are highly sensitive to the detailed nature of those changes and that presently we have only low confidence in numerical projections of changes in flood magnitude or frequency resulting from climate change.

(5) A few important things to remember about global warming

Please read this before commenting about my views about global warming and climate change. It also has links to the key posts on the FM website on this topic.